The optimum control of heating and cooling of enclosed spaces, such as residential structures, has become a highly developed art. This art has been refined by the control of the temperature in such structures by the means of real time operated thermostats that utilize microcomputers or microprocessors that have extensive memories. With the use of microcomputers and memories, it is possible to build a thermostat that is capable of accurate control and one that is capable of generating substantial energy savings. The energy savings are accomplished in the heating mode by the thermostat setting back the building temperature at night or when the building is not occupied, and then returning the temperature control to a normal temperature at a preselected time. Energy can also be saved in a cooling mode by setting the building temperature up during an unoccupied time, and then returning the temperature setting to a lower temperature for normal occupation of the building.
These changes save energy by lowering the need for energy consumption when a building is not occupied, or when the occupants can tolerate the variation from a normal temperature. These functions have been long recognized and utilized with various types of clock-operated thermostats. Technology has progressed to the point where prior temperature cycles for the building are stored in the microcomputer's memory for use in controlling the future temperature cycles within the building. This type of control, while being generally energy efficient, has a serious flaw when the heating and/or cooling source is a heat pump that utilizes auxiliary heat. Typically, a heat pump will have a very good cost efficiency, and the auxiliary electric heat will have a relatively poor cost efficiency. In present day equipment that uses setback for energy savings with heat pumps, the heat pump and electric auxiliary heat both come on during the pick up cycle. This may be inefficient from an energy standpoint as the heat pump alone might be capable of bringing the building back to a normal temperature in a reasonable period of time without the use of auxiliary heat. This function has been recognized, and outdoor sensors are sometimes incorporated in these systems. When an outdoor sensor is used, the outdoor temperature at which auxiliary heat is enabled is arbitrarily selected which may result in less than efficient use of the auxiliary electric heat. In order to obtain maximum house shell savings and system efficiency, a heat pump with auxiliary heat must be operated with a judicious use of the auxiliary heat.